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Reading up on how lasers work, I found out that there are lots of different materials that can be used as laser medium, from noble gas ions and halogens to metal atoms to organic dyes. So far I have understood that to get laser action your laser medium needs to have several energy levels, with a high level that is metastable so that the spontaneous transition from the high to the low level is slow, so that the high energy state can accumulate, and a population inversion can form. For many substances these energy levels are electronic states, but vibrational states also work (e.g. CO2 lasers).

What I'm wondering, is if in principle any element from the periodic table can be used as a laser medium, or if there are some constraints on that. Do the types of energy levels and metastable states that are needed for laser action exist for all atoms, or are they rare? Are there any rules that an element or molecule must meet to be able to lase? If not, are there heuristics that predict that? And do these rules or heuristics also generalize to molecules and other types of lasers where it is not single atoms that are doing the lasing?

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    $\begingroup$ A Vantablack laser has suddenly popped into my head as a Covid-19 project, but, no, I should really do those shelves I've been putting off first. :-) $\endgroup$ May 19, 2020 at 14:18
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    $\begingroup$ As an experienced laser physicist once told me, if pump something hard enough it will lase. It may not be what you started out with once you've pumped it, but... $\endgroup$
    – Jon Custer
    May 19, 2020 at 14:22
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    $\begingroup$ So if it doesn't lase, pump it harder. Sounds like a meme I should have seen on T-shirts or something. Good thing my first sub-question specifies atoms, I guess you can pump them pretty hard before you induce nuclear disintegration. 😆 $\endgroup$
    – JanKanis
    May 19, 2020 at 14:37

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I will not give a too detailed answer, just partial, and although I am a laser scientist, I am not a laser material scientist.

For example, solid metals have partially occupied conduction bands. This means it's hard to excite them in the typical way you do dielectric materials, like ruby lasers. And I do not believe it to be possible to have a laser out of solid silver. But then metal vapors can be ionized and excited in different manners, which can lead to lasers (for example HeHg or HeCd lasers).

In principle, you would find that any material can lase, as there are a multitude of excited states which can be accessed, if enough effort is put in place, but perhaps not every material can lase in every state of matter. Its just that the effort highly outweighs the returns. You would not choose to deliver electricity to your house via small watch batteries locked in a park 20km away, which need replacement of 1000 random cells per day, but it's one way of doing it.

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